1. Field of the Invention
This invention relates to cooling air flowpaths for high temperature turbines.
2. Description of the Prior Art
In the field of gas turbine technology a great deal of research has been directed towards improving thermodynamic efficiency by operating gas turbine engines at higher temperatures. To permit the engines to operate at the new gas stream temperatures which are higher than internal materials can normally tolerate, considerable effort has been devoted by engine manufacturers to development of advanced and highly efficient methods of cooling internal turbine hardware. In particular, components in the hot gas stream, such as turbine nozzle guide vanes and rotor blades, are exposed to very severe temperatures and require significant amounts of cooling air.
In early designs, cooling methods of these high temperature parts were limited to transferring of heat from turbine part surfaces to low temperature parts by conduction. When air cooling methods were introduced they were originally limited to blowing cooling air across the face of turbine discs.
In contrast, today's more advanced turbine technology utilizes hollow turbine nozzle vanes and blades with high pressure cooling air being drawn through internal passages within the vanes and blades. This internal cooling method has proven to be very effective, but it requires delivery of significant amounts of cooling air to the blades and vanes. The normal delivery path for blade cooling air presents a significant mechanical obstacle. The problem is the air must be drawn from a nonrotating source of compressed air, usually the compressor outlet, through a rapidly rotating turbine shaft. Forcing the air through this rotating shaft causes aerodynamic losses which translate into an increase in the temperature of the air. Because the air is used to cool hot turbine parts, any increase in the temperature of the air means that a greater volume must be used and this results in a decrease in engine efficiency.
Recently, turbine engines have employed nozzles that are aimed in the direction of turbine rotation to accelerate this cooling air in the direction of rotation for ease of entry into the rotating shaft. The nozzle directs the air through simple holes drilled through the shaft wall. To reach the turbine blades, the air must pass through these holes in the shaft, then move radially outward into the hot turbine blades.
While this improvement has been employed with reasonable success, further gains are achievable. Analysis has shown that after exiting the turbine nozzle, the cooling air has a very high velocity that is actually greater than the rotational speed of the turbine shaft. This means the cooling air has a tangential velocity that is greater than the tangential velocity of the shaft. When the air slows down to pass through the shaft holes, a large pressure loss occurs. This represents an unrecoverable loss of energy. Also, if the air is flowing to a smaller radius, as in passing under a turbine disc, its tangential velocity will increase even more, sometimes actually causing an acoustic resonance. Recently turbine engines have employed flat radial vanes inside the turbine shaft to remove residual tangential velocity in the cooling air. This has been successful in eliminating acoustic resonance but has further increased the aerodynamic losses in the cooling airflow system. These potential problems can be avoided by eliminating any excess tangential velocity of the cooling air before the air enters the turbine shaft.
It is, therefore, an object of the present invention to direct nonrotating cooling air into a rotating turbine section of a gas turbine engine without significant pressure losses and with lower resulting cooling air temperatures and reduced cooling airflow requirements.
It is another object of the present invention to direct cooling air into a rotating turbine section without excessive tangential velocity.
These and other objects will become more readily apparent from reference to the following description taken in conjunction with the appended drawings.